From my prior experience I have seen similar issues with firefox. 

This issue arise when firefox process large HTTP post request.

Please try using firefox for reproducing the issue and get the output for below :

show np 1 me-stats -shttp

Look for counter value 

Exceed max buffer errors:                     16901    <<<<<  Check if the counter increase after the test. 

you can try few times and in case if you see the value increasing for  “ Exceed max buffer errors”

I believe if you do the following and test that should fix it. 

configure set header-maxparse-length 65535 and set max-parse-time 65535 and set content-maxparse-length 65535

host1/Admin(config)# parameter-map type http http_parameter_map
host1/Admin(config-parammap-http)#
host1/Admin(config-parammap-http)# set content-maxparse-length 65535
host1/Admin(config-parammap-http)# set max-parse-time 65535
host1/Admin(config-parammap-http)# set header-maxparse-length 65535
host1/Admin(config-parammap-http)# length-exceed continue

Apply the above in multimatch policy :

appl-parameter http advanced-options http_parameter_map

Hope that helps.